Solid polymer electrochemical fuel cells comprise a plurality of unit cells each comprising an electrolyte membrane in the form of an ion exchange membrane and a catalytic electrode and a porous carbon electrode which are disposed one on each side of the electrolyte membrane. Hydrogen supplied to the anode of the fuel cell is converted into hydrogen ions on the catalytic electrode, which move through the electrolyte membrane that has been humidified to an appropriate extent toward the cathode of the fuel cell which is made of porous carbon. An oxygen containing gas or air is supplied to the cathode electrode to generate water through a reaction between the hydrogen ions and the oxygen on the cathode electrode. Electrons which are generated at this time are led to an external circuit for use as electric energy as a direct current. Such a fuel cell is disclosed in Japanese laid-open patent publication No. 6-20713. In view of the fact that when the water is supplied to humidify the electrolyte membrane, the water may be collected as a drain on the surfaces of the separators depending on the conditions in which the fuel cell operates, the disclosed fuel cell has parallel grooves defined in the separators for supplying a fuel gas and an oxygen containing gas, respectively, the grooves being directed downwardly in the direction of gravity for draining the collected water in order to enable the solid polymer electrolyte membrane to generate electric energy at a sufficiently high level.
Specifically, since the operating temperature of fuel cells of the type described above is relatively low, water generated by a reaction between the fuel gas and the oxygen containing gas and also water added to the fuel gas or the oxygen containing gas to humidify the electrolyte membrane tend to be condensed in the gas passages in the separators, closing the gas passages thereby to lower the performance of the fuel cells.
As shown in FIG. 31 of the accompanying drawings, if a fuel cell 4 has a number of fuel cell cells 2 stacked along the direction of gravity, then water droplets are collected in regions (e.g., regions 6a or 6b) where the flow of a fuel gas or an oxygen containing gas that has been humidified, greatly lowering the performance of those fuel cells 2 which are positioned adjacent to the fuel cells 2 including the regions 6a or 6b as compared to the other fuel cells 2.
In the other fuel cells 2, since the fuel gas and the oxygen containing gas flow in directions perpendicular to the direction of gravity, condensed water is liable to be collected in portions of the fuel cells 2, so that voltages generated by the fuel cells 2 will vary from each other. Furthermore, because the water in the gas passages is temporarily discharged, it is not possible to prevent voltages generated by the fuel cells 2 from varying from each other.
The fuel cell disclosed in Japanese laid-open patent publication No. 6-20713 has such a structure that the fuel gas and the oxygen containing gas flow in directions perpendicular to the direction of gravity along the solid polymer electrolyte membrane, the anode electrode, and the cathode electrode, and cooling water flows perpendicularly to the fuel gas and the oxygen containing gas flow. While this structure is effective to alleviate shortcomings caused by unstable voltages that possibly occur due to generation and elimination of condensed water, it has been confirmed with the disclosed structure that the current density is temporarily increased owing to a temperature rise at the outlets of the fuel cells.
More specifically, it has been confirmed that, as shown in FIG. 32 of the accompanying drawings, when a fuel gas such as a hydrogen containing gas and an oxygen containing gas such as an oxide gas flow in a direction perpendicular to the direction of a cooling water flow with respect to a solid polymer electrolyte membrane 12 that is sandwiched between an anode electrode 8 and a cathode electrode 10, the temperature of the fuel cell 2 is higher downstream than upstream with respect to the gas flows. Particularly, the temperature is higher at the outlet of the cooling water flow than at the inlet of the cooling water flow.
The same phenomenon is observed when the fuel gas and the oxygen containing gas flow parallel to the direction of the cooling water flow with respect to the solid polymer electrolyte membrane 12 as disclosed in Japanese laid-open patent publication No. 5-144451, for example. This is shown in FIG. 33 of the accompanying drawings. In the arrangement shown in FIG. 33, the temperature of a lower portion of the fuel cell 2 is higher than temperature of an upper portion thereof. The phenomenon indicates that the heat generated by a heating reaction is subjected to a heat exchange with the gases and the heat caused by a contact resistance, etc. is subjected to a heat exchange with the gases on the cathode electrode 10 and the anode electrode 8, causing the temperature of the gases to rise progressively toward the outlet of the fuel cell 2, with the gases heating the electrodes themselves. As a result, each fuel cell 2 suffers a temperature distribution between upstream and downstream portions of the gases, with the result that the voltage generated by the fuel cell 2 suffers a distribution. Accordingly, the output voltage produced by the fuel cell 2 is not stable, shortening the service life of the fuel cell 2 itself. If the fuel cell 4 comprising fuel cells 2 each having a temperature distribution is used as a power source for motor vehicles, then a complex control process will be required to control the running of the motor vehicle.
There is known a fuel cell comprising a first unit cell, a fuel gas supply means, a cooling plate, an oxygen containing gas supply means, and a second unit cell that are successively stacked in order to remove heat produced upon generation of electric energy, as disclosed in Japanese laid-open patent publication No. 5-190193. In the disclosed fuel cell, the cooling plate has cooling water passages defined therein, and the first and second unit cells are cooled by the fuel gas supply means and the oxygen containing gas supply means. The cooling efficiency of the surface of the cooling plate which is held against the fuel gas supply means is higher than the cooling efficiency of the surface of the cooling plate which is held against the oxygen containing gas supply means.
According to the above prior art, the cooling efficiencies of the anode and cathode electrodes are set to optimum levels by positioning the cooling water passages of the cooling plate closely to the fuel gas supply means, or providing individual cooling water passages respectively in the fuel gas supply means and the oxygen containing gas supply means, or using cooling members having different thermal conductivities respectively with respect to the fuel gas supply means and the oxygen containing gas supply means, or making a fuel gas passage member thinner than an oxygen containing gas passage member.
With the above prior art, the fuel gas supply means, the cooling plate, and the oxygen containing gas supply means are disposed as a separator interposed between the first and second unit cells. Consequently, the separator is made up of many components, has a large thickness, and does not make the fuel cell compact as a whole. Another problem is that the fuel cell is heavy in its entirety because the separator is made up of many components.
It has been proposed to use a porous material as a separator for directly humidifying a fuel gas and a solid polymer electrolyte membrane in order to keep the solid polymer electrolyte membrane and an ionic conduction component in a constantly wet state at all times (see Japanese laid-open patent publication No. 6-231793).
The presence of a contact resistance in a fuel cell structure increases an internal ohmic loss, thereby lowering a voltage between its terminals. It is necessary to impart desired tightening forces to the fuel cell structure for the purpose of reducing the contact resistance.
The above separator, however, fails to impart tightening forces directly to the fuel cell structure for structural reasons, and hence the fuel cell needs to have a structure dedicated to produce tightening forces. As a consequence, the fuel cell is constructed of an increased number of parts, large in size, and heavy in weight.
It is an object of the present invention to provide a method of controlling a fuel cell which comprises a number of fuel cells each employing an electrolyte membrane, stabilizes the output voltage of each of the fuel cells, increases the service life of the fuel cells, and is simple in structure and inexpensive to manufacture.
Another object of the present invention is to provide a fuel cell which has anode and cathode electrodes that can be set to optimum cooling efficiencies, respectively, is made up parts that are not increased in number, can be made compact and light.
Still another object of the present invention is to provide a fuel cell which is simple in structure and can uniformize the temperature of an electric generation section easily and accurately.
Yet still another object of the present invention is to provide a fuel cell whose electrolyte membrane can directly be humidified, to which desired tightening forces can be imparted, which is simple in structure, and which has many functions.